Servo - December 2016

e know what you might be thinking — all I want for Christmas is a Twin Tweaks article on something other than a pneumatic punkin chunker. We hear you. We’ll make it a New Year’s resolution. Yes, this is another pneumatic cannon project, but it’s different from our previous efforts by at least an order of magnitude of awesome. It’s like the end of a Mythbusters episode, where if they can’t get the Christmas tree to ignite from its lights or clean out a cement truck with dynamite, they strap copious amounts of C- 4 to everything and blow it to kingdom come.

Here, however, our goal is sort of the opposite — we want to push the limits of our pneumatic air cannon punkin chunker design, but we did not want to blow ourselves up. As we alluded to last time, high pressure large-scale projects come with their own unique set of design considerations, and they require specialized parts that we couldn’t simply scrounge up from the garage. The weeks leading up to our big build day felt like the 12 Days of Bot-mas, with boxes full of heavy steel goodness showing up on our doorstep almost every day. Would Santa bring us what we needed for our cannon? Or, did our single-minded focus on building a better punkin chunker relegate us to the naughty list?

On the First Day of Bot-mas, McMaster Brought to Me ...

As we alluded to, we wanted to build a high pressure version of our PVC punkin chunker because it would be awesome and it would be a good way to explore the special challenges of an extreme project. We were sticking with the tried and true trident design of our PVC chunkers,

but scaled down slightly. Instead of 4” diameter tanks and barrel, we opted for 2. 5” tanks and a 3” barrel. This would significantly save on the weight of our chunker while still allowing for good performance. As detailed back in the October 2015 issue, we created an isothermal model of cannon muzzle speed to better understand the impact of various design considerations on the performance of the cannon. Our chief performance objective was maximizing muzzle speed — we figured that was a good proxy for chunked punkin distance. We determined that having a tank volume to barrel volume ratio of at least 2:1 for a given pressure ensured that we didn’t leave too much muzzle speed on the table (tank to barrel volume ratios above two gave diminishing returns). A larger barrel diameter than tank diameter would, of course, decrease our tank to barrel volume ratio, but by messing with the isothermal model we discovered that giving up a little on the volume ratio allowed us to gain a lot more in other areas. A bigger barrel would better allow us to accommodate a bigger projectile (punkin), and a bigger heavier projectile would actually have significantly higher impact force than a smaller lighter one. That would make for a more spectacularly exploding punkin, and just like the Mythbusters loading up a cement truck with high Twin Tweaks by Bryce Woolley and Evan Woolley